Frequency shift relaying protective network with noise control

ABSTRACT

A FREQUENCY SHIFT OPERATED RELAYING SYSTEM HAVING MEANS FOR PREVENTING TRIPPING IN THE PRESENCE OF NOISE AND IN WHICH MEANS IS ALSO PROVIDED TO PERMIT TRIPPING IN THE PRESENCE OF NOISE WHEN THE TRIP FREQUENCY IS MAINTAINED FOR A MINIMUM INTERVAL INDICATIVE OF THE EXISTENCE OF A FAULT CONDITION.

United States Patent 36; 340/171, 310, 216, (inquired) [56] References Cited UNITED STATES PATENTS 2,860,324 11/1958 Berger et a1. 340/310 3,144,585 8/1964 Blakemore 317/28 3,312,866 4/1967 Rockefeller... 317/28 3,327,170 6/1967 Sonnemann 340/310X Primary Examiner-James D. Trammeil AttorneysA. T. Stratton, C. L. Freedman and John L.

Stoughton ABSTRACT: A frequency shift operated relaying system having means for preventing tripping in the presence of noise and in which means is also provided to permit tripping in the presence of noise when the trip frequency is maintained for a minimum interval indicative of the existence of a fault condition.

l2 2'0 24 FAULT M: I scmlo'r 7i 1 F Vj PONSIVE us TRIGGER |o DEVICE cncun' CLAMP I n Iii}; I161 OSCILLATOR I I I QSCILLATQR mo KEYE R rmysmwenwuplsa 1] a TELEPHO TERMNAL"A" I CENTRAL OFFICE A L I J.f 'l I.- 50

""I 5| OSCILLATOR AMPLIFIER 4 4' I B I I 'I: n2

II II II A CENTRAL QFEICE"B T ELEPHON E TERMINAL B r I E I RECEIVER I NETWORK AT II II .56 B

. I Q L l PATENTEU JUN28 l9?! SHEET 1 OF 8 EEEETQEtEmEuP PATENTED JUN28 I9?! 8; 588,610

sum 3 [IF 8 3 1/ FREQUENCY SHlFT OSCILLATOR AND KEYER /TRANSMITTER AMPLIFIER I TRANSMITTER FILTER I24 T fi---- PATENTED JUN28 187i DECIBELS V.D.C. INPUT TO KEYING CIRCUIT I I I FIG. 5

| l I ms 295] FIG. 7A.

RECEIVER LIMITER AND SIGNAL SUPERVISORY MODULE PATENTEU JUN28 I9?! SHEET 5 [IF 8 PATENIEB JUII28 IQTI SHEET 8 [IF 8 DECIBELS FREQUE NCY SHIFT DISCRIMINA TOR AND DC AMPLIFIER HIGH LINE-LEVEL TRIP BLOCK BACK LASH BLOCK RELEASE NORMAL SIGNAL LEVEL RECEIVER DYNAMIC RANGE= IO.5dB

BLOCK RELEASE LOW SIGNAL LEVEL TRIP BLOCK MINIMUM S/ N RATIO= 9dB FOR SECURE OPERATION 3CIB MARGIN 7* NOISE TRIP BLOCK FIG. 7B

BLOCK RELEASE PATENTED JUN28 I97! SHEET 7 0F 8 mime: Eowimw m @902 92 533 mil vmO PATENTED JIJN28 I9?! SHEET 8 UF 8 ll I! I! I00 300 500 700 900 H0O I300 I500 FREQUENCY FIG. 9

CONTROL NETWORK G NETWORK I ue as e FREQUENCY SHIFT RELAYING PROTECTIVE NETWORK WITH NOISE CONTROL BACKGROUND OF THE INVENTION In prior art frequency shift relaying systems operating on different frequencies of audio tones to provide guard and trip frequencies, means was provided to prevent undesired tripping of the circuit breaker and thereby the deenergization of the protected line section when no fault existed due to noise which may be present in the channel which transmits the tone signal from one relaying station to a second relaying station. This resulted in the inability of the primary protection being unable to trip the breaker in the event of a fault and the reliance upon the backup protection to actuate the breaker during the interval-in which the normal tripping means was prevented from operation due to the existence of noise on the channel. Such channel could be a public utility transmission line between the two central offices, a microwave connection, a carrier current connection, etc.

In accordance with this invention, means is provided in such a frequency shift relaying system to trip the breaker in the event of a fault even in the presence of a higher noise level. In the event of a fault in the protected line section, the trip frequency will continuously exist until the fault is cleared. Therefore when the trip frequency or tone continues for a predetermined minimum time interval, indicative of a fault, tripping will occur. Should the trip frequency be caused by channel noise such frequency will not last as a continuous signal for this predetermined minimum interval.

It is an object of this invention to provide means which will permit tripping of a breaker by a frequency shift relaying system even though a noise condition exists which would prevent such tripping of the breaker in prior art system.

Other objects of the invention will be apparent from the description, the appended claims and the drawings, in which drawings:

FIG. 1 is a block diagram showing a frequency shift relaying system embodying the invention;

FIG. 2 is a block diagram of the receiver end of the relaying network of FIG. 1 showing certain blocks thereof in somewhat greater detail;

FIG. 3 is a schematic diagram showing circuitry suitable for use in certain of the blocks of FIG. 1;

FIG. 3A is a chart showing certain operating characteristics of the apparatus of FIG. 3;

FIG. 4 is a schematic diagram showing circuitry for use in certain other ofthe blocks of FIG. 1;

FIG. 5 is a chart showing a typical characteristic of a channel filter for use in connection with the receiver shown in F lIG.

FIG. 6 is a schematic diagram showing circuitry suitable for use in certain ofthe blocks shown in FIG. 2;

FIG. 7 is a schematic diagram showing circuitry which may be used in certain other of the blocks of FIG. 2;

FIG. 7A is a chart showing certain operating characteristics of the discriminator of FIG. 7;

FIG. 7B is a chart showing certain operating characteristics of the relaying system of FIG. 2 when certain noise levels occur in the channel between station A and station 8;

FIG. 8 is a schematic diagram showing circuitry which may be used in the blocks shown in FIG. 2;

FIG. 9 is a chart showing typical operating characteristics of the noise filter illustrated in FIG. 2; and,

FIG. I0 is a schematic diagram of circuitry for use in FIG. 2.

Referring to the drawings by characters of reference, the numeral 1 indicates generally an oscillator and keyer network at station A; shown in block form in FIG. 1 and in schematic form in FIG. 3. The numeral 2 designates a transmitter-amplitier network at station A; shown in block form in FIG. 1 and in schematic form in FIG. 4.

The numeral 3 indicates the receiving network at the station B; shown in block form in FIG. I and in more detailed blocks in FIG. 2. The signal generated by the networks 1 and 2 at the switching station A is connected to a telephone terminal 32 also at station A. This terminal 32 connects with a local telephone company central office A by a local telephone cable pair 33. The signal thus supplied to the central office A is transmitted to the telephone central office B by any usual channel, wired or unwired and diagrammatically shown by the dash line 59. The central office 58 connects by means of a local cable pair 56 with a telephone terminal 55 at the switching station B. This terminal 55 connects with the receiving network 3. I

The frequency shift oscillator comprises a Schmidt trigger circuit 4 which is controlled from suitable apparatus 6 which responds to the existence of fault current and/or voltage at the switching station A. This apparatus is not shown in detail since any conventional apparatus for use in frequency shift protective relaying systems may be used. This apparatus 6 controls the voltage applied to the Schmidt trigger circuit 4. When voltage applied to the Schmidt trigger circuit is below a critical value, the clamp network 8 is connected in one winding 11 of a transformer 12. The other winding 13 of the transformer 12 is connected to the input terminals 16 of the oscillator 14 and is shunted by a capacitor 18. When the clamp network 8 effectively open-circuits the capacitor 10, the oscillator 14 generates'the transmitted signal at guard frequency at its output terminals 16. With the clamp network 8 conducted the capacitor 10 is effective to change the generated signal to trip frequency. The Schmidt trigger circuit 4 controls the operating condition of the clamp network 8 as indicated in the chart of FIG. 3A.

The output terminals 20 of the oscillator 14 are connected to the input terminals 24 of the transmitter-amplifier 2 which has its output terminals 26 connected to the input terminals 28 of the primary winding 29 of the conventional transformer 30 at the telephone terminal 32. The terminal 32 is closely adjacent to the location of the switching station A of the protected transmission line. The secondary winding 34 of transformer 30 includes an adjustable tap 35 and is connected to energize a pair of busses 3637 which energize the cable pair 33 which interconnects the telephone terminal 32 with the telephone central office 38. The conventional mutual inductor 40 of terminal 32 has the end terminals of its winding 41 connected between the busses 36-37 and has a center tap connection 42 connected to earth through the gap 44.

The cable pair 33 at the central office 38 is shunted by means of the usual mutual inductor 46 which has its end terminals separated from the cable pair 33 by the usual spaced carbon blocks 47 and 48. The center tap connection 50 of the inductor 46 is connected to earth at the location of the telephone office 38.

The remote switching station B is provided with a receiving network 3 shownin greater detail in the block diagram of FIG. 2. The receiving network 3 is coupled by the pair of conduction 5 and the usual transformer 54 to the cable pair 56 at the telephone terminal 55. The telephone cable pair 56 connects the terminal 55 with the telephone central office 58 which serves the telephone terminal 55.

Upon the occurrence of a fault which includes zero sequence current at the station A, the potential of the earth 43 at the telephone terminal 32 will vary considerably with respect to the potential of the earth 51 at the telephone central office 38 at the frequency of the transmission line. This difference in potential of the earth at locations 43 and 51 causes to flow through each half of the winding 41. Since the potential induced in the two winding portions by this current flow is not exactly balanced, a potential having a fundamental frequency of the power line and rich in harmonics will be induced into the cable pair 33 and will be transmitted along with the oscillator signal to the receiver 3 as noise. As will be explained in more detail below, and as shown in greater detail in FIG. 7 this noise will prevent the substantially instantaneous operation of the breaker trip control 60 to prevent false actuation by noise but will permit operation of the trip control when the trip frequency continues for a predetermined minimum time interval which may for example be 25 milliseconds. The breaker trip control 60 is converted through a control network 61 to the output terminals 62 and 63 which are energized respectively by the signal generated by the transmitter 2 and by the noise responsive network.

The receiver network 52 comprises a plurality of modules 64, 65, 66 and 67. The modules 64 and 66 provide a filter band pass characteristic as shown in FIG. and a combined band pass and notch filter as shown in FIG. 9 respectively. Preferably suitable filters are used as for example those sold under the trade designations HB-63l00 (FIG. 5) and HIS-55183 (FIG. 9) by RFL Industries, Inc., Boonton, NJ. (formerly Radio Frequency Laboratories). Any other suitable modules which provide the desired output characteristics may be used such as a combination of simple filters and logic circuits. FIG. 5 shows the filter characteristic of two filters such as 64 and represents the relative band pass characteristics when multiplexing of the channel is used to transmit two intelligence signals. Only one filter module 66 is used to respond to noise and the noise responsive module 67 would be used in conjunction with each of the sets of intelligence modules corresponding to modules 65 and 66 which are associated with each of the added filter modules which are tuned to the multiplexed signal.

The output terminals of the filter module 64 are converted to the input terminals 68 of the receiver limiter and signal supervisory intelligence module 65. The frequency of the signal supplied to this module 65, as illustrated in FIG. 5, is limited to substantially the frequency range of the signal between the trip and guard frequencies. The line 70 shows the center frequency, the line 70T trip frequency and line 70G guard frequency. The lines 72, 72T and 72F show the similar signal frequency for an adjacent intelligence signal.

The module 65 contains an amplifier section 74 which supplies the amplifier signal to an amplitude limiting network-.76, the output of which is connected to the input 78 of the discriminator and DC amplifier intelligence module 66. The module 66 includes a frequency shift discriminator network 80 having its input energized from the terminal 78 and it is supplied to the clamping network 82. The'network 82 is in turn connected to the trigger network 84 and this in turn to clamping network 86. The output of the network 86 is amplified by a DC amplifier 88 and supplied to the before mentioned output terminal 62 of the intelligence module 66. The terminal 62 is connected through one input terminal of an AND network 90 and to one terminal of the timing network 90 while the output terminal 63 is connected through a NOT network 94 to the other input terminal of the AND network 90. When a signal of trip frequency occurs at the input terminals 68 of module 65 an output signal is provided at the output terminal 62 of the module 66 which will, in the absence of a signal at terminal 63, cause the AND network 90 to energize its output terminal 95. The terminal 95 is connected to one input terminal 96 of the OR network 97. With the terminal 96 energized, the OR network 97 will actuate the breaker trip control 60.

The receiver 3 includes the noise module 67 which has two sets of input terminals 98 and 99. The terminals 98 connect directly with the conductor 5 through a resistor while the terminals 99 connect with the conductors through the filter module 66 and a resistor. The signal at the terminals 98 passes through a sensitivity control 180, is amplified by amplifier I01 and the amplified signal is rectified in the rectifier I02 and supplied to the trigger network 103. Similarly the signal at the terminals 99 is passed through a sensitivity control 104, an amplifier 105 and a rectifier 106 to the trigger network 103. The trigger network 103 is connected through a delay network 107 to the amplifier 108 which has its output connected to the output terminal 63. 4

The strength of the output quantity of the rectifier 102 is a measure of the magnitude of the total energization of the terminals 5 throughout the total range of the energy frequencies and is representative of the noise level as well as the level of the intelligence signals. As is indicated by FIG. 9, the filter module 66 passes only a limited band of relatively low frequency energy which as indicated may be between 300 Hz. and 950 Hz. Assuming a 60 Hz. power transmission line, the filter 66 will pass generally from the 5th to the 15th odd harmonic frequencies. The band-pass filter or noise filter 66 provides a high impedance to a frequency which is substantially midway between the 5th and 15th harmonics. A suitable frequency for this narrow band high impedance is 595 Hz. This notch 110 is provided to attenuate the 595 Hz. frequency supplied by the oscillator amplifier combination 112 located at the telephone terminal 32. If no frequency distortion is present in the 595 Hz. signal between the terminal 32 and the receiving network 3 the filter 66 will attenuate the 595 Hz. signal and except for a small increase in the overall noise level, the module 67 will be uneffected. The 595 Hz. signal does however, indicate a frequency translation in the connecting circuit and if the 595 Hz. signal as received is not substantially at the frequency which is attenuated by the notch 110, the result is a noise to which the module 67 will respond to prevent false actuation of the breaker trip control 60 which might otherwise occur due to a frequency swing or distortion in the interconnection from the oscillator 112 to the receiving network 3.

It is desirable to monitor the connection between the stations A and B to insure the absence of any substantial amount of frequency distortion. In the event that the distortion was in a direction to increase the frequency, the trip frequency could be ineffective to cause a tripping or if multiplexing were used the guard frequency 720 might even increase sufficiently to provide the trip frequency 70T and falsely trip the breaker in a direction to decrease the frequency. The guard frequency 70G might decrease sufficiently to provide the trip signal 711T. When noise is received at the input terminals 98 and/or 99 of module 67 of sufficient intensity, the module 67 provides an output signal at its output terminal 63 which actuates the NOT circuit 94 and removes the signal normally provided thereby to the AND circuit 90. With the NOT circuit so energized, the intelligence module 66 is ineffective to cause the AND circuit to actuate the OR circuit 97. g

If however the energization of the output terminal 62 is maintained for the minimum interval for which the first timer 114 of the timing network 92 is set to provide the delay before actuation of the second timer 116 thereof, the OR circuit 90 is actuated to actuate the breaker trip control 60. If the output terminal 62 is not maintained energized for the entire 25 millisecond delay of the timer 114, even though only momentarily interrupted, the timer 114 will reset and the signal at terminal 62 must again exist for the minimum timing period of the timer 114. In order to insure operation of the breaker trip control 60 by the OR circuit 97 rapidly upon timing out of the timer 114, the timer 116 is provided with substantially no delay time upon energization. In order to maintain the 0R circuit 97 energized for a time period subsequent to any resetting of the timer 114 after it has timed out, the timer 116 provides a time delay sufficient to provide a positive actuation of the breaker trip control 60.

While the various modules 1, 2, 65, 66 and 67 may take various forms, suitable circuitry therefor is illustrated in FIGS. 3, 4, 6, 7 and 8. Even though these schematics are believed to be self-explanatory to those skilled in the art in view of the foregoing description a brief description of the operation of each is believed to be in order.

The oscillator and keyer module (FIG. 3), during normal operation of the transmission line protected by the relaying network does not receive sufficient voltage at its input terminals 118 to render the transistor Q1 conducting with transistor 02 nonconducting. The transistor Q2 conducts in response to base current which flows from the positively energized bus 131 through the resistor R1 base to emitter in the transistor 02 and resistor R2 to the negative bus B2. The value of the resistance of the resistor R2 is low (it may conveniently be 47 ohms) so that when the transistor 02 conducts through a circuit extending from the bus Bl through a resistor R3 and the collector to emitter in the transistor Q2 substantially the entire potential drop between the busses B1 and B2 appears across the resistor-R3 (which, for example, may be of 510 ohms). With the transistor Q2 conducting, no base current will be supplied to the transistors Q3 and Q4 so that the circuit extending from the winding 11 of the transfonner 12 through the capacitor and the resistor R4 is open circuited. Under these conditions, the transfonner 12 in cooperation with the capacitor 18 tunes the multivibrator type oscillator 14 to generate the guard frequency. This frequency may be anywhere from 1360 to 3060 Hz. or such other range depending upon'the desires of the designer. The output signal is supplied by flt transformer 12 to the output terminals 20 of the oscillator 14 to the sensitivity control 120.

As indicated in FIG. 3A, the transistors Q3 and Q4 of the clamp conduct to cause the oscillator 14 to provide the trip frequency when the voltage between the busses B1 and B2 approaches l6.volts.'The transistors Q3 and Q4 will be maintained conducting until the potential between the busses B1 and BZis'reduoed by the apparatus 6 to a value which may be substantially 12% volts. When the potential between busses B1 andB Z approached 16 volts, the Zener diode Z1 began to conduct base current from the bus B1 through the resistor R5, base to emitter in the transistor 01, and resistor R2 to the bus B2. With the transistor Q1 conducting, the base drive current for thetransis'tor O2 is shunted and the transistor Q2 ceases to conduct and base current flows from the bus B1 through the resistorR3to the transistors Q3 and Q4. These transistors conduct and effectively insert the capacitor 10 into the circuit of the transformer 12 to tune the oscillator 14 to generate the trip frequency. Depending upon the bandwidth of frequencies which is desired the trip frequency 70T may be anywhere from 170 to 340 Hz. below the guard frequency 706.

The input'terminals 124 of the transmitter module 2 (FIG. 4) areconnected through a gain or sensitivity control 125 to the transistor ()5 and then to the transistors 06 and Q7 to transmit the signal to the telephone terminal 32 through a filter 126. The filter 126 is of course optional but it does benefit the transmission of the signal. I

The receiver limiter and signal supervisory module 65 of the receiving network 3 (FIG. 6) has its input terminals 68 connected to the channel filter 64. The amplifier 74 is connected to the input terminals 68 through a sensitivity control 12.7. The output of the amplifier 74 is supplied through the resistor R10 and capacitor C10 to the base of the transistor Q10. The transistor Q10 has its collector connected to bus B10 connected to the positive potential supply and its emitter connectedthrough a-resistor R11 to the negative bus B11 which is connected to the negative terminal of the potential supply. The limiter 76 of the module 65 is provided with a second transistor 011 having its emitter connected through a resistor R12 to the bus B10 and its collector connected through the resistor R11 to the bus 1311. The base of the transistor Q11 is connected through a resistor R13 to a bus B12 which is maintained at a desired negative potential with respect to the bus B10 by means of the Zener diode 210 and voltage dropping resistor R14. The transistors Q10 and Q11 and their associated circuitry limit the magnitude of the voltage swings supplied to the input terminals 78 of the frequency shift discriminator and DC amplifier module 66.

The module 66 is shown in detail in FIG. 7. The discriminator 80 may take any of usual forms common to the art and is illustrated in FIG. 7 as comprising a pair of transformers T20 and T21. One isconnected in a tuned circuit tuned to the trip frequency. The other is connected in a tuned circuit tuned to the guard frequency. The outputs of the transformers T20 and T21 are applied through resistors R20 and R21 across a potentiometerRZZIhaving its movable balancing tap connected to a common connection between the transformers T20 and T2]. The transformers T20 and T21, in the usual manner, provide the output characteristic as indicated by the curve of FIG. 7A. In the in t c'e shown, the center frequency 70 is 2295 Hz.

and there is a frequency difference of 170 112. between the guard 706 and trip 70T frequencies. When guard frequency is received a negative voltage appears at the base of the transistor 020 of the clamping network 82 whereby the transistor Q20 is maintained in its nonconducting state. When, however, the trip frequency is received the potential supplied to the base becomes positive and the base current flows to render the transistor Q20 conducting. When transistor Q20 conducts, formerly conducting transistor Q21 becomes blocked and thereafter base current flows from the positive bus B20 through the base emitter circuit of the transistor Q22. Transistor Q22 conducts and base current flows through the resistor Q23 to raise the potential of the output terminal 62.

In the prior art devices, the module 67 was connected to render a normally nonconducting transistor Q24 conducting in the event of noise in the transmission link. When conducting, transistor Q24 maintains the base potential of the transistor Q22 sufficiently low to prevent the conduction of transistor Q22 in response to any operation of the transistor Q21. in accordance with this invention however, this interconnection isnot used. As discussed above the alteration of the actuation of the breaker trip control 60 by the module 66 is accomplished by connecting the breaker trip contact 60 to the module 66 and the noise supervisory module 67. As described above, the presence of noise prevents immediate operation of the breaker trip control by the module 66.

The line level and noise supervisory module 67 is shown in greater detail in FIG. 8. The input terminals 99 are connected to the input of the amplifier 105 through the sensitivity adjustment control 104. The output of the amplifier 105 is supplied to the transistor 030 which controls the degree of energization of a transformer T30, the output of which is supplied to a trigger circuit comprising the transistors Q31, Q32 and Q33 of a trigger circuit similar to the trigger circuit described above in connection with the module 1. The output of the trigger circuit 103 is connected through a delay network 107 which comprises the transistors Q34, Q35 and thereafter through the amplifier 108 which comprises the transistors Q36 and Q37. When the transistor Q37 conducts, the potential of the output terminal 63 of the module 67 is maintained substantially that of the positive bus B30 and when energized operates the NOT network 94 to remove the input signal normally supplied by the NOT network to the AND network 90.

The module 67 also has its input terminals 98 connected directly to the line 5 and through a sensitivity control device 100 to the amplifier 101 which like the amplifier 105 has its output connected to the input terminal 128 of the trigger network 103. The sensitivity adjustments 104 and 100 are preferably adjusted, as indicated in FIG. 78, so that the noise sampled by the noise filter, when it reaches an 18 decibel magnitude, will energize the output terminal 63 of the module 7.

FIG. 10 illustrates, schematically, a circuit which may be used in the module or network 61. The input terminal 130 is connected to the output terminal 62 of the intelligence module 66 and through resistors 132 and 134 to the common terminal 136 between the diodes 138 and 140 of the AND network 90. The common connection between the resistor 132 and 134 is connected to the common connection 143 between resistors 142 and 144. A diode 146 is connected between this common connection 143 and the base of a transistor 148 having its emitter connected to the negative bus 150 and its collector connected through the resistor 142 to the connection 143. A timing capacitor 152 is connected in shunt with the collector-emitter circuit of the transistor 148. A transistor 154 has its base emitter circuit connected through a Zener diode 156 across the capacitor 152. When the charge on the capacitor 152 reaches a critical charge (as determined by the R/C of its charging circuit), the diode 156 will conduct base current and the transistor 154 will conduct.

When the potential of the terminal 143 is made positive with respect to the bus 150 due to presence of a signal at trip frequency, the capacitor 152 charges to time out the predetermined minimum time interval during which the signal at trip frequency must continuously exist to actuate the breaker trip control 60. if the signal at terminal 130 is removed the transistor 148 will conduct and discharge the capacitor 152 to its initial condition.

The collector of the transistor 154 is connected to the common point 158 of a pair of resistors 160 and 162 which are se-' ries connected between positive and negative busses 164 and 166. A transistor 168 has its collector connected to bus 164 through a voltage dropping resistor 170, its emitter connected to bus 166 and its base connected to point 158. The collector is also connected to the output terminal 171 of the timer 116 through two parallel circuits 172 and 173. Each of these parallel circuits 172 and 173 includes a diode having its anode connected to the collector and a resistor connecting the cathode to the terminal 171. A capacitor 174 and a discharging resistor 175 are connected in parallel between the common connection of the diode and resistor of the parallel circuit 173 and the bus 166.

When the transistor 154 is not conducting base current flows in the transistor 168 and the potential of the terminal 171 is maintained substantially at the potential of the negative bus 166. When the transistor 168 is rendered nonconducting due to the timing out of the timer 114 and the conduction of the transistor 154, the potential of the collector of the transistor 168 rises and the conduction through the circuit 172 elevates the potential of the terminal 171. The conduction through the diode of circuit 173 rapidly changes the capacitor 174. When the transistor 168 subsequently becomes conductive, the capacitor 174 will maintain the potential of the terminal 171 elevated for a desired time interval as determined by the R/C constant of the resistor 175 and capacitor 174.

The base of the transistor 176 of the NOT circuit 94 is connected to receive base drive current from the input terminal 178 through a diode 180 and a resistor 182. The emitter is connected to the cathode of the diode 140 of the AND circuit 90 and the collector is connected to the negative bus 184.

in the absence of noise, the module 67 will be ineffective to supply base drive to the transistor 176. in the absence of a signal at trip frequency, the intelligence module 66 will not energize the terminal 130 sufficiently to either case the timer 114 to time or the AND network 90 to actuate the R network 97 for energization of the breaker trip control 60. In the presence of a signal at trip frequency, the intelligence module 66 will elevate the potential of the terminal 130 and that of the terminal 96 of the 0R network 97 through the diode 138 and resistors 132, 134 and 186 to cause conduction of the transistor 188 of the breaker trip control 60.

If however, noise actuates the noise responsive module 67, the base current will flow through the transistor 176 and the transistor 176 will conduct and maintain the potential of the terminal 136 at a potential sufficiently close to that of the negative bus 184 to prevent the transistor 188 from conducting. Under these conditions, the capacitor 152 will charge at a rate determined by the resistor 142. At the end of the timing interval of the timer 114, assuming the signal at trip frequency is a time signal and the modules 65 and 66 are not being falsely actuated by noise at the trip frequency, the timer 114 will cause the transistor 188 to conduct and thereby trip the breaker (not shown) even though the noise persists.

My invention will prevent the substantially instantaneous operation of the breaker from occurring in the presence of noise in the transmitted signal and thereby prevent false breaker operation by such noise as was accomplished by the prior art. My invention additionally permits breaker operation in the presence of noise when a fault occurs in the potential line section without the reliance upon backup line protection.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

1 claim: What is claimed and is desired to be secured by U.S. Letters Patent is as follows:

1. Apparatus for use in a frequency shift relaying system wherein the system utilizes a control signal transmitted at first and second frequencies to said apparatus and which signal is subject to noise superimposed thereon comprising, a first network for actuation by said signal and having a first output when said signal is at said first frequency and a second output when said signal is at said second frequency, a second network for actuation by said noise and having a third output when said noise is present, third and fourth networks, each of said third and fourth networks having first and second input terminals and an output terminal, means connecting said first network to said first input terminal of said third network whereby said just-mentioned first input terminal is energized by said first and second outputs of said first network, means connecting said second network to said second input terminal of said first network whereby said just-mentioned second input tenninal is energized by said third output, means connecting said output terminal of said third network to said first input terminal of said fourth network, a fifth network, means connecting said fifth network between said first network and said second input terminal of said fourth network, said third network being effective to provide a first control signal to said first input terminal of said fourth network solely in the absence of said third output to said second input terminal of said third network, said fifth network being effective to supply a second control signal to said second input terminal of said fourth network solely subsequent to a predetermined minimum time interval during which said second output is continuously supplied thereto, said fourth network being effective to provide a third control signal at its output terminal when at least one of said first and said second control signals are supplied thereto.

2. The combination of claim 1 in which said third network is effective to supply said first control signal solely when said first network is supplying said second output.

3. The combination of claim 2 wherein said fifth network maintains said second control signal for a predetermined minimum time interval subsequent to termination of said second output.

4. The combination of claim 1 in which said fifth network includes first and second timing devices, said first timing device being connected between said first network and said second timing device and effective to actuate said second timing device solely subsequent to said predetermined minimum time interval during which said second output is continuously supplied to said fifth network, said second timing device being connected between said first timing device and said second input terminal of said fourth network and effective to substantially immediately apply said second control signal upon actuation by said first timing device and to maintain said second control signal for its said predetermined minimum time interval after actuation of said second timing device by said first timing device is terminated.

5. The combination of claim 3 in which said third network comprises an AND network and a NOT network, said AND network having a pair of input terminals and an output terminal, one of said input terminals of said AND network being said first terminal of said third network, said output terminal of said AND network being said first terminal of said AND network, said NOT network being connected between the other of said input terminals of said AND network and said second input terminal of said third network.

6. In a relaying system for protecting a section of a power transmission line extending between a first station and a second station, signal receiving equipment at said second station, said equipment having a pair of input terminals adapted to receive a variable frequency signal from said first station, switch actuating means, connecting means connecting said equipment to said actuating means, said receiving apparatus comprising first and second modules, said first module being connected to said input terminals and effective in response to the reception at said terminals of a first frequency of said signal to provide a first output and to provide a second output in response to the reception at said terminals of a second frequency of said signal, said second module being connected to said input terminals and responsive to frequencies at said terminals other than said first and second frequencies to provide a third output, said connecting means including a first network, and a timing device and a second network, said first network having first and second input terminals and an output terminal, means connecting said first module to said second input terminal of each first network and said second module to said first input terminal, said first network being effective to provide a first operating signal-at its said output terminal solely when said second output is supplied by said first module and said third output is supplied by said second module, said second network having first and second input terminals and an output terminal, means connecting said output terminal of said first network to said first input terminal of said second network and said output terminal of said timing device having an input and an output terminal, means connecting said input terminal of said timing device to said first module and said output terminal of said timing device to said second input terminal of said second network, said timing network being effective to provide a second operating signal at its said output terminal solely subsequent to the existence of said second output for a predetermined minimum time interval, said second network being efiective upon the existence of at least one of said operating signals to provide an actuating signal to actuate said actuating means.

7. The combination of claim 6 in which said connecting means includes means to maintain said actuating signal for an interval subsequent to the occurrence of said second operating signal and to the disappearance of said second output.

8. The combination of claim 6 in which said timing device includes signal maintaining means to maintain said second operating signal for an interval subsequent to the disappearance of said second output.

9. The combination of claim 8 in which said timing device includes a first timer to time out said minimum time interval and said signal maintaining means includes a second timer, said first timer being operable to return to an initial condition to time said minimum time each time said second output is removed, said second timer being effective to initiate said second operating signal substantially immediately subsequent to the timing out of said minimum time interval by said first timer. 

